The present invention generally concerns production of proteins and/or xanthophylls from corn.
Corn is one of the major crops in the United States. About 25% of corn is converted to food, feed, and industrial products. Ethanol production from corn by the dry-grind process has increased exponentially recently and is expected to reach 7.5 billion gallons by 2012. At present, a dry-grind ethanol plant typically produces three products: ethanol, carbon dioxide, and distillers dried grains with solubles (DDGS). Since operating costs presently are almost equal to the revenue from ethanol, any profit is derived from the coproducts, as well as from subsidies and tax waivers. The dry-grind ethanol process currently is a low-profit operation, and there is a need for new technologies and additional coproducts to improve its profitability.
Protein forms about 10% of the dry weight of corn. It is composed of zein (a highly hydrophobic protein, soluble in isopropanol or ethanol) and glutelin (soluble in aqueous alkaline solutions), with lesser amounts of globulins and albumins. Zein is unique in that it is insoluble in water except in the presence of alcohols or high concentrations of alkali or anionic detergent. It is a protein with several existing and potential applications in the food, pharmaceutical, and biotechnology industries. As nonlimiting examples, zein has great potential in biodegradable films and packaging materials, chewing gum, pharmaceutical gel tablets, and a variety of other markets, as identified by R. Shukla and M. Cheryan, “Zein: The industrial protein from corn,” Industrial Crops and Products, 13: 171-192.
However, it is desirable to substantially improve the purity of the obtained zein to command a higher price and volume. Further, it is desirable to reduce the cost of production.
Various methods of producing zein from corn have been discussed in articles such as those by R. Shukla and M. Cheryan, above, and in several patents. Most of these methods use aqueous solutions of ethanol to do a first extraction of the zein from ground, flaked, or otherwise size-reduced whole corn or corn processing by-products such as corn gluten meal. However, the ethanol solvent co-extracts several other compounds such as xanthophylls, polyamines, lipids, and other compounds that have not yet been identified. Thus, the various processes are distinguished mainly by the subsequent methods of separation and purification to increase purity of zein from about 10-50% to about 90%.
For example, U.S. Pat. No. 6,433,146 to Cheryan, incorporated herein by reference, describes a method for extracting zein from corn using aqueous ethanol, followed by a separation (e.g., filtration or centrifugation) to remove suspended particles, and typically followed by one or more membrane separation steps using ultrafiltration and/or nanofiltration membranes to purify the zein by removal of low-molecular weight impurities. In preferred methods, the ethanol is recycled. U.S. Pat. No. 6,846,909 to Mairal also describes a process for purifying zein using membranes.
U.S. Pat. Nos. 6,602,985 and 6,610,831 describe a process that uses water to remove water-soluble components from ground corn or corn gluten meal, followed by ethanol extraction of zein and treatment of crude zein with activated carbon. However, zein yield in such a process can be as low as 25%, since zein may be bound to the carbon.
U.S. Pat. No. 5,580,959 to Cook et al. describes a process of purifying zein using enzymatic starch hydrolysis, alkaline treatment, ethanol extraction, and activated carbon adsorption. U.S. Pat. Nos. 5,342,923, 5,367,055, and 5,510,463 to Takahashi disclose using acetone, hexane, and other organic or hydrocarbon solvents to remove oil, color pigments and odor compounds from corn gluten meal followed by extraction of zein using aqueous ethanol. In these patents, the protein is exposed to enzyme or high pH or organic solvents or strong adsorbents, which may change the natural conformation and/or composition of the protein. This, in turn, may limit the potential uses of the extracted zein.
U.S. Pat. No. 4,624,805 to Lawhon teaches extraction and purification of corn endosperm proteins using ultrafiltration. However, the product is a mixture of zein and glutelin.
Generally, conventional purification methods usually do not provide high yields and purity simultaneously, nor do they produce a zein that is devoid of xanthophylls.
One of the coproducts in preferred embodiments of the present invention is xanthophylls such as lutein, zeaxanthin, and beta-cryptoxanthin. Lutein and zeaxanthin have numerous potential health benefits, including mitigating age-related macular degeneration, cardiovascular damage, and certain forms of cancer. Xanthophylls cannot be synthesized by the human body and must be obtained from foods such as fruits, vegetables, and eggs, or from dietary supplements containing chemical- or fermentation-derived compounds. Among natural sources, egg yolk and corn contain the highest molar levels of lutein and zeaxanthin (more than 85% of the total carotenoids). Most sources of xanthophylls other than corn are rich in either lutein or zeaxanthin, but not both.
The major commercial source of xanthophyll today is marigolds. U.S. Pat. No. 6,262,284 to Khachik describes extraction of lutein using an alkaline organic solvent of tetrahydrofuran containing ethanol and KOH maintained at pH 12. U.S. Pat. No. 6,911,564 to Khachik describes methods of chemical conversion of lutein to anyhydroluteins. In both patents, Khachik describes purifying lutein by column chromatography using n-silica gel and mobile phases such as C5-C7 hydrocarbons or petroleum ether in combination with acetone or methyl ethyl ketone or ethyl acetate or tetrahydrofuran or C4-C6-ethers. However, the xanthophylls in marigolds are in an ester form and require additional steps to obtain the pure form. The product is also quite unstable and cannot be processed into very high concentrations of the xanthophylls. Further, marigolds produce mainly lutein and not zeaxanthin, and the methods taught by these patents use a complicated series of steps and expensive solvents.